In the cleaning unit of a combine harvester several sieves are arranged executing oscillating movements, which are designated according to their spatial arrangement as preliminary sieve, upper sieve and lower sieve. In operational use of the combine harvester it is necessary for optimal cleaning of the threshed material, to adjust the width of a sieve mesh according to the quantity of material to be threshed, the nature of the material to be threshed and the degree of moisture present in the material. Generally it is the case that with an increase in grain size and with rising moisture the width of a sieve's mesh must be greater. Such devices for adjusting the width of a sieve's mesh are known already in a multitude of variants.
One of the most frequently encountered variants is shown in the operating instructions from the NEW HOLLAND firm with the printed note “Print no. 84017041, 02/99”, which possesses a manually operable setting lever, which can be swung in a horizontal plane and can be locked in several intermediate positions in a predetermined swinging range. This swinging motion is transmitted via a connecting rod to the adjusting rail of the louvres, so that they are twisted together with the louvre wires, which serve amongst other things as bearing points. The consequence of this is a raising up or lowering of the louvres, through which the width of the sieve's mesh can be increased or diminished. These devices are fixed at the rear end of the sieve frame, viewed in the direction of the flow of threshed material, through which a preliminary sieve consisting of one piece is equipped with a device for adjusting the width of the sieve mesh, which is accessible by hand via an opening in the right hand side wall of the combine harvester. The upper sieve and the lower sieve consist of two sieve sections arranged next to each other, both of which possess such a device for itself, which is attached likewise at the rear end of the sieve box and which are to be operated by approaching from the rear on the combine harvester within the operator's height of reach.
A disadvantage of devices thus constructed and arranged for adjusting the width of the sieve's mesh is that through this the costs for manufacturing the combine harvester are increased, since every sieve and indeed every sieve section is equipped with a separate device. Furthermore it is to be criticised that after removal of a sieve for the purpose of cleaning or for exchanging a sieves after installation of the cleaned or exchanged sieve the previous size of the sieve mesh must be reset again, which is associated with an increased expenditure of time. With the employment of this device on a preliminary sieve another complication is added in that one the one hand accessibility is poor and on the other hand the set width of the sieve's mesh cannot be checked exactly.
A further variant of a device for adjusting the width of a sieve's mesh was made known with DE-OS 1 952 808. This is attached to a sieve, which is surrounded by a sieve frame, which is subdivided in its breadth by means of longitudinal ribs into three equal sieve sections. In each section of the sieve there are an equal number of louvres, whose horizontally arranged louvre wires are carried in the left and right lateral carrier of the sieve frame and in the longitudinal ribs so that they can twist. For adjusting the width of the openings in the sieve's louvres each louvre wire possesses a kink, which per section of the sieve is carried in a common longitudinally running adjusting rail. In order to be able to adjust all the louvres simultaneously, in the rear transverse support of the sieve box a tubular transverse shaft extending over the entire width of the sieve is arranged in bearings so that it can twist, which in the central region of each sieve section possesses shallow lugs directed downwards. A connecting rod engages with one of its ends with each of these lugs while its other end it is suspended in the longitudinally running adjusting rails. To actuate it the transverse shaft possesses on one side a positively locking inset in the form of a square drilling, into which is inserted a square shaft which is rigidly joined to an adjusting lever. This adjusting lever works together with a locking segment fixed to the sieve box, which runs in the arc around the longitudinal axis of the transverse shaft and is equipped with several drilled notches, into which an indexing bolt fixed to the adjusting lever can engage optionally.
Also with this device for adjusting the width of a sieve's mesh there is the disadvantage that they represent a cost-increasing factor, since every sieve is fitted with a separate device. Likewise the lack of a capability for reproducing a previously set size for the sieve's mesh after cleaning or an exchange of sieves is to be criticised. Finally such a device is not to be recommended for employment on a so-called preliminary sieve because of poor accessibility and limited capability for checking the set width of the sieve's mesh.